Need bug help

Hundreds, if not thousands (I didn’t count 'em all) of these guys all over basement carpet. All dead.

Walk out basement, finished, radiant heat in floor (not in service at time of inspection). Moisture meter showed less than 3% moisture (most <2%) on drywall. carpet not wet, stained nor musty odors. I figured these guys need the mositure, why did they come into the house?

House has been vacant for 6 months or so - client says he didn’t see any of them in July when he first looked at the place.

Any ideas?